Method of removing air and water contaminants trapped with the crystal structure of sodium chloride



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This invention relates to a process for removing the last traces of air and water from salts used as constituents of electrolytic baths for the production of reactive refractory metals of groups IV, V, and VI of the periodic table.

One commonly employed method of producing reactive refractory metals such as titanium, zirconium, and the like is by electrolysis of the metallic compound dissolved in an electrolyte consisting of fused alkali or alkaline earth metal halides. Electrolytes commonly utilized in this connection are calcium chloride, mixtures of calcium and magnesium chlorides, mixtures of calcium chloride, sodium chloride, and potassium chloride, as well as sodium and potassium chlorides, either alone or in combination.

Major difficulties in the production of reactive refractory metals arise due to their easy contamination by oxygen, moisture, and nitrogen introduced into the electrolytic cells with the electrolyte, either as air voids or entrapped with mother liquor in the crystallized salt. Of these impurities, oxygen is perhaps the most deleterious since its presence renders the subsequent purification of the metal very difiicult and, in some cases, virtually impossible. A water content as low as 0.2 percent of the Weight of a particular electrolytic salt, a content actually quite low when one considers the hygroscopic nature of some compounds found satisfactory as major constituents of electrolytic baths (for example, calcium chloride), is sulficient to prevent the production of ductile, high-purity metal in large crystal form, as is required by increasingly more rigorous metallurgical standards. Nitrogen and nitrogen-bearing contaminants are known to exert an embrittling effect on refractory metals.

Several attempts to overcome these difliculties have been suggested and tried. Efforts to eliminate the occlusion of water and air by modified recrystallization procedures were unsuccessful. In this connection, attempts were made to produce monolithic crystals by decreasing solubility through the addition of acid or methanol, and by the slow evaporation of the salt solution under vacuum. In allcases the products obtained had microscopic internal chambers containing either air or brine.

It has also been proposed to dry electrolytic salts at 200 C. at atmospheric or reduced pressure. If the total volume of the electrolyte is quite small, the amount of water remaining after the above treatment does not appear detrimental, but if the bath volume is increased by as little as a weight factor of 2, the quality of the metal produced decreases very sharply. Completely inadequate results are obtained by this method with cells containing more than about 5 pounds of electrolyte.

Another procedure involves a pre-electrolysis stage in which the cell is operated under reduced voltage for a considerable length of time, moisture being removed by electrolysis of the oxygen-bearing compounds, by absorption by the metal deposited during that period, and by the heat of the bath. This process requires the discarding of the metal first produced, and is, therefore, economically impractical.

There is a great need for an efficient and cheap method which will remove the contaminants and thereby eliminate their deleterious effects on the reactive metals produced by fusion electrolysis.

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It is accordingly a primary object of this invention to provide an economical and efficient method for drying electrolytes suitable for use in the fusion electrolysis of these metals. Other objects are to provide a more efficient method for drying alkali and alkaline earth metal halides prior to their use as components in electrolytic baths from which reactive metals are to be deposited, and to provide alkali and alkaline earth metal halides having a sufficiently low elemental oxygen and nitrogen content to permit the deposition of reactive metal of exceptional purity from an electrolytic bath of such salts.

The above objects are achieved by heating the electrolytic salt to a temperature between its melting point and boiling point in an environment free of deleterious contaminants such as oxygen, nitrogen, and water and maintaining said evironment at a pressure not exceeding atmospheric pressure during the heating period to purify said salt and render it substantially free of said Water and air constituents.

In order to achieve optimum results, the salt may be preheated for a length of time at a relatively low temperature to remove some of the contained water prior to the more complete removal described above. This heating step may be accomplished either under vacuum or at atmospheric pressure. A standard drying oven and a temperature of about 100 C. may be used, for example.

One of the advantages of this invention is the fact that the process is quite flexible insofar as temperature, pressure, and environment are concerned. The temperature must be above that required to melt the salt and below its boiling point. The lower the absolute pressure, the more thorough the removal of contaminants. However, if the surface of the salt is flushed with an inert gas, the pressure may be as high as atmospheric with satisfactory results.

The time required for the removal of contaminants depends upon a great number of variables. For any particular case, the point at which the purging is substantially complete may be determined by observing the vapor pressure, in the case of vacuum heating, or by measuring the dew point of the efiluent gas, in the case of an inert gas flush. For example, if a salt is heated through its melting point while contained in a vacuum of approximately 25 microns of mercury, absolute pressure, the release of Water, air and other contaminants formerly entrapped in the crystal structure will be followed by an increase in pressure-to 500 microns, for example. As the contaminants are removed by the vacuum pump, the pressure will slowly return to a value approximating the original. At that point, the purification step is substantially complete.

Similarly, if the dew point of an eflluent inert gas is measured, a parallel phenomenon will be observed whereby the dew point will increase to a high value and will then decrease when removal is complete, often to values as low as F. or F.

It will be understood that so many variables influence the actual removal times that precise limitations cannot be attached thereto. The particular vacuum pumping system employed, for example, as well as deposits in the piping, dryness of the inert gas source, and the type of chamber employed will all have some bearing on the time. In the following examples, times have been chosen based on either dew point measurements or pressure measurements.

Several modifications of the basic invention are possible. In one embodiment, the electrolyte material is maintained at an absolute pressure not exceeding 300 microns for a period of 4 to 6 hours, the temperature being held at 50 C. to C. above the melting point of the material treated.

An alternative method consists in initially heating the material to a temperature slightly below its melting point in an atmosphere of inert gas at a pressure not greater than 3 centimeters of mercury, and preferably not exceeding 100 microns; subsequently increasing the temperature slightly over the melting point of the treated material, and adjusting the pressure to a value not exceeding 3000 microns, preferably while flowing through the heating chamber dry, inert gas at a rate of about 3 to 5 liters per minute.

A further modification of the invention is to first heat the electrolytic material or salt under vacuum at a temperature well below its melting point, and subsequently to heat the material to a temperature slightly above its melting point at a pressure not exceeding atmospheric in the presence of an inert gas, passing such gas through a melting chamber at a flow rate of 8 to 12 liters per minute. In accordance with this variation of the invention, the salt may be heated even at atmospheric pressure.

The first two modifications of the invention will remove moisture at a faster rate than is the case with the third procedure. Investigation has shown that both vacuum treatments will satisfactorily remove moisture in a period of 5 to 7 hours, and that 13 to 15 hours are required for the third method, when treating about 17 pounds of sodium chloride. In the three methods, the length of time of treatment is proportional to the quantity of material being purified, and to the flow rate of the inert gas. Close control of temperature is required to reduce volatilization of the salt with consequent risk of condensation within, and plugging of any associated piping. Any inert monatomic gas such as helium, argon or neon may be used.

The instant invention has been successfully applied to a considerable number of electrolytes, and the following specific examples are given for a clearer understanding of the invention, and to illustrate the principles and broad application of its methods. As is well known, the hardness of reactive refractory metals is sharply increased by the presence of small amounts of such impurities as oxygen, nitrogen, carbon, and hydrogen, so that hardness values are generally used as an indication of metal purity. Accordingly, hardness values will be given in these examples for later comparison with that of the metal obtained from salt treated according to prior art.

Example 1 Approximately 17 pounds of reagent grade sodium chloride were pretreated by vacuum drying for 23 hours at 200 C. and an absolute pressure of 3 cm. mercury. It was then heated under pressure not exceeding 300 microns, and usually maintained near 100 microns, within the temperature range of 850 C. to 900 C. for 6 hours. No inert gas was passed through the treatment chamber during this treatment. The salt so treated was used as an electrolyte in the deposition of tantalum metal. The metal thus obtained was found to have a hardness of 77 Brinell.

Example II Seventeen pounds of reagent grade sodium chloride were vacuum dried for a period of 23 hours at 200 C., and a pressure of 3 centimeters of mercury. Following this treatment the salt was heated to 770 C. under a pressure of 25 to 50 microns, and when the temperature had become equalized throughout the mass, the cell pressure was allowed to increase to 2000 microns by regulating the addition of argon to a flow rate of 4 liters per minute. The temperature was then raised to 825 to 850 C., and maintained within this range for a period of 6 hours. The dew point of the efiluent gas issuing from the treating chamber was initially 65 F., but decreased to 20 F. by the end of the treatment. The salt so treated was collected and used as an electrolyte in the production of tantalum metal by electrolysis. The hardness of a 100 gram arc-melted specimen of the metal obtained using this electrolyte was 99 Brinell.

Example III About 17 pounds of reagent grade sodium chloride were dried for 23 hours at 200 C. at a pressure of 3 centimeters of mercury. It was then heated in an enclosed chamber to a temperature between 800 C. and 850 C. for a period of 14 hours. Approximately 8400 liters of argon were passed through the chamber at atmospheric pressure during this period, and, based on dew point measurements, 16 grams of water or 0.2 percent of the original salt weight were removed. The salt thus treated was used in an electrolytic bath for the production of tantalum metal, which exhibited a hardness of Brinell.

A similar quantity of reagent grade sodium chloride was also conventionally dried for 24 hours at a temperature of 200 C. under a pressure of 3 centimeters of mercury. An electrolytic bath for the production of tantalum was prepared from this salt, and the metal deposited therefrom was found to have a hardness of 253 Brinell, a value much higher than that exhibited by the metal produced from electrolytes treated according to the method of this invention.

Another important effect achieved by this purification procedure, and which is believed to be directly related to the reduction of the impurity content of the metal was the increase in crystal size of the deposited metal. Metal obtained by the use of impure electrolytes is finely divided and powdery, whereas the metal produced from an electrolyte purified according to the method of this invention is in the form of dendritic crystals, which are usually from 1 to 2 millimeters in diameter and 4 to 5 millimeters in length. This increase in crystal size is desirable since large crystals are not as affected by atmospheric oxidation, and may be remelted to secure massive metal more easily than the finely divided material. Another significant advantage possessed by the larger crystal form is the greater ease and rapidity with which spent electrolyte can be leached from the metal. Such leaching may be quickly and simply accomplished with hot water without danger of deleterious surface oxidation. The need for acids or special chemicals to prevent hydrolysis and surface oxidation as required in the case of powdery metal is accordingly eliminated.

The operation of the cell is improved by greater salt purity. The cell atmosphere is clear, and less salt than usual is transported from the bath by vaporization and entrainment in the etliuent gases. The efiiciency of current utilization and the metal recovery are both increased since there is no waste of energy through the electrolysis of oxygen-bearing compounds.

From the foregoing it will be apparent that the invention provides a new method for purifying electrolytes and improves upon prior methods which have been technically and economically impractical. The electrolytes so produced are free of the least traces of air and water, and their use in electrolytic baths for the production of reactive metals results in a very pure, highly ductile product having large crystal size.

As used in this specification, the term periodic table refers to a periodic chart of the type found at pages 56 and 57 of Langes Handbook of Chemistry, eighth edition, 1952, published by Handbook Publishers, Inc.

This is a continuation-in-part of my copending application Serial No. 427,886, filed May 5, 1954, and entitled Improvement in Process for Fusion Electrolysis of Reactive Metals, and now abandoned.

What is claimed is:

l. A method of removing water and air constituents entrapped within the crystalline structure of sodium chloride intended for use as electrolyte components in the fusion electrolysis production of reactive refractory metals of groups IV, V, and VI of the periodic table "t" at a.

which comprises heating said halide to a temperature between its melting point and boiling point in an environment free of deleterious contaminants, and maintaining said environment at a pressure not exceeding atmosphen'c pressure during the heating period to purify said halide and render it substantially free of said water and air constituents.

2. A method of removing water and air constituents entrapped within the crystalline structure of sodium chloride intended for use as electrolyte components in the fusion electrolysis production of reactive refractory metals of groups IV, V, and VI of the periodic table which comprises heating said halide to a temperature between its melting point and boiling point in an environment free of deleterious contaminants, maintaining said environment at a pressure not exceeding atmospheric pressure during the heating period, and flowing a dry, inert gas across the surface of said halide to purify said halide and render it substantially free of said water and air constituents.

3. A method of removing water and air constituents entrapped within the crystalline structure of sodium chloride intended for use as electrolyte components in the fusion electrolysis production of reactive refractory metals of groups IV, V, and VI of the periodic table which comprises heating said halide to a temperature between its melting point and boiling point in an environment free of deleterious contaminants, maintaining said environment at a pressure not'exceeding atmospheric pressure during the heating period, and flowing a dry, inert gas selected from the group consisting of helium, argon, and neon across the surface of said halide to purify said halide and render it substantially free of said water and air constituents.

4. A method of removing water and air contaminants entrapped within the crystalline structure of sodium chloride intended for use as electrolyte components in the fusion electrolysis production of reactive refractory metals of groups IV, V, and VI of the periodic table which comprises initially heating said halide to a temperature below its melting point and at a pressure not exceeding atmospheric pressure to remove substantial amounts of water and air therefrom, further heating said metals of groups IV, V, and VI of the periodic table which comprises initially heating said electrolyte component to a temperature below its melting point at a pressure not exceeding 3 centimeters of mercury, increasing the temperature above said melting point, decreasing the pressure to a value not exceeding 3000 microns, passing a dry, inert gas over said electrolyte component to purify said electrolyte component and render it substantially free of Water and air constituents.

6. A method of removing water and air constituents entrapped within the crystalline structure of sodium chloride intended for use as electrolyte components in the fusion electrolysis production of reactive refractory metals of groups IV, V, and VI of the periodic table which comprises placing said electrolyte component in a heating chamber, reducing the pressure to a value not exceeding 300 microns, heating said electrolyte component at a temperature to C. above its melting point until measurements indicate substantially no efllux of moisture, and cooling said electrolyte component whereby the purified electrolyte component is obtained in the solid state.

References Cited in the file of this patent UNITED STATES PATENTS 2,564,498 Nisbet Aug. 14, 1951 2,762,684 Wainer Sept. 11, 1956 OTHER REFERENCES Mellors Modern Inorganic Chemistry," 1939, Longmans, Green and Co., N.Y., pp. -161.

Kroll et al.: Bureau of Mines Report of Investigation #4915. I 

1. A METHOD OF REMOVING WATER AND AIR CONSTITUENTS ENTRAPPED WITHIN THE CRYSTALLINE STRUCTURE OF SODIUM CHLORIDE INTENDED FOR USE AS ELECTROLYTE COMPONENTS IN THE FUSION ELECTROLYSIS PRODUCTION OF REACTIVE REFRACTORY METALS OF GROUPS IV, V, AND VI OF TTHE PERIODIC TABLE WHICH COMPRISES HEATING SAID HALIDETO A TEMPERATURE BETWEEN ITS MELTING POINT AND BOILING POINT IN AN ENVIRONMENT FREE OF DELETERIOUS CONTAMINANTS, AND MAINTAINING SAID ENVIRONMENT AT A PRESSURE NOT EXCEEDING ATMOSHERIC PRESSURE DURING THE HEATING PERIOD TO PURITY SAID HALIDE AND REENDER IT SUBSTANTIALLY FREE OF SAID WATER AND AIR CONSTITUENTS. 